When using chromatography for separating individual chemical compounds present in a mixture, the choice of a stationary phase is important to get good resolution, i.e., good separation of the compounds. There is a wide variety of stationary phases available, which have different affinities to different compounds. More specifically, it is usually the surface of the stationary phase that interacts with the components of the mobile phase in a desired manner, i.e., the stationary phase acts as an adsorbent material.
Silica, or other inorganic oxides, can be used as the base material for stationary phases in chromatography. In order to functionalise the support material, a specific organic compound may be attached to the surface by reaction with e.g. silanol groups on a silica surface. By varying the compound to be bonded to the surface, various chromatographic behaviours can be achieved. However, silica and other inorganic oxides may start to dissolve at high pH, causing leakage of inorganic material, usually referred to as inorganic leakage. To avoid inorganic leakage, it is important to cover as much of the support surface as possible. The support is suitably a porous material having pores of various sizes. The main part of the surface area of the support material is the area within the pores. It is therefore important that this part can be coated with, e.g., a polymer, to avoid inorganic leakage and to give good resolution.
The chromatographic stability of an adsorbent material is also an important factor. By chromatographic stability is herein meant the ability of the adsorbent material to keep its selectivity and retention over a period of use.
Polymers may also be attached to a support surface by first attaching a monomer onto the support surface followed by further polymerisation. H. Engelhardt et al., Chemically Modified Surfaces, Proc. Symp 4th, Elsevier, 1992, 225–241, discloses a method of coating a solid support where a vinylsilane is bonded to a silica surface followed by co-polymerisation of acrylamide onto the vinyl group of the covalently attached vinylsilane. However, this will give a predominantly tentacle-type coating where most polymer chains are attached in a single point, with each polymer chain extending from the support as a “tentacle”, giving a both chemically and mechanically unstable attachment. Generally, the chemical bond between a coating and e.g. silica is often unstable, especially at low pH, which may give leakage of organic material, usually referred to as organic leakage. Also, when polymerising vinyl monomers onto a solid support, remaining monomers may be present on the support surface giving chemical instability. Alternatively, a preformed polymer may be attached onto a support surface in multiple points. WO 98/27418 A1 discloses a method of coating a silica support by first binding a coupling agent onto the support surface and thereafter binding a preformed polymer to the coupling agent. The preformed polymer comprises a totally saturated carbon chain with leaving groups. It is a rather complicated method requiring several process steps. Kurganov et al, Journal of Chromatography 261 (1983) 223–233, discloses a method for bonding a copolymer of styrene and methylvinyldimethoxysilane onto a surface of silica. However, the reactivity of methylvinyldimethoxysilane is much lower than for styrene, which will favour homopolymerisation of styrene giving only few silane monomer parts in the copolymer, thereby effecting the number of linkages to the silica in a disadvantageous way. Alternatively, a chloromethylated polystyrene is reacted with aminopropyltriethoxysilane and thereafter bonded to a surface of silica. However, the basic nitrogen in the amino group in the spacer between the polymer backbone and the silica is not an inert group and may take part in unwanted reactions. U.S. Pat. No. 4,914,159 discloses a process for immobilising copolymers of (meth)acrylamides onto a silica gel. A (meth)acrylamide monomer is copolymerised with a silylating agent such as methacryloyloxypropyltrimethoxysilane.
There is a need of further improved adsorbent materials and improved methods of coating solid supports for making adsorbent materials. It is therefore an object of the present invention to provide an adsorbent material for chromatography which has high chromatographic stability, which gives excellent resolution of chemical compounds to be separated from a mixture and which has a high degree of surface coverage of the solid support. There is a further object of the present invention to provide a method for preparing an adsorbent material for chromatography, which comprises few steps, which gives an adsorbent material with high degree of surface coverage of the solid support, and which enables flexibility in providing adsorbent materials with different chromatographic behaviour.